2 Answers
2

is it better to alias them and setup some kind of rewrite rules to make the content look like separate domains

No. If you do this you will have duplicate content issues as the same content can be retrieved from more then one URL. If you have multiple domains you will want to do a 301 redirect from the alternate domains to the primary domain. This will prevent duplicate content issues and if someone ever tries to use an alternate domain they will find the primary instead.

is it better to set them up as landing pages with slightly different content & then redirect to the main site?

No. Those pages will have no SEO value due to having little content and no links to them themselves. So links from those pages will be worthless. You're life will be much easier if you just redirect the alternate domains to the primary domain from the get go.

If you do want to keep two separate sites (and sometimes there are good reasons to do this then my understanding is that in this scenario you should use:

<link rel="canonical" href="url-to-primary-version-of-content">

This instructs the search engine that this content is a duplicate and that the primary (cannonical) source of this information exists at the listed url.

My understanding is that this practice was put in place so that you CAN have duplicate content so long as you correctly reference its canonical source. A great example of where this could be used is for reuters news articles, which are published in many places.